1. Field of Invention
The present invention relates to wireless communication receivers, and amongst other things to wireless communication receivers having signal processing capability for performing beam forming, antenna diversity, as well as other functions.
2. Discussion of Background
Wireless modems are used to interconnect computers or LANs using radio waves. Radio signals have advantages and disadvantages over hardwire systems. The wireless advantages include quick deployment, higher data rates than conventional telephone lines and the ability to serve customers independently of the traditional infrastructure.
The disadvantages of wireless transmission in the range of the MMDS/ITFS (multi-channel multi-point distribution system/instructional television fixed service) channels include fading and multi-path.
Wireless receivers utilize several techniques to overcome channel impairments like fading and multi-path. Once class of techniques to reduce fading and multi-path are referred to as beam forming. Beam forming techniques increase the antenna gain, and more importantly increases the antenna directivity while also allowing controllable direction selectivity. This enables reduction of external interference and improvement of the robustness against multi-path.
Another class of techniques to reduce problems associated with fading and multi-path are referred to as antenna diversity. Antenna diversity uses two or more antennas that are shifted in space by one or more wavelengths of the transmitted carrier frequency. The spatial shifting can influence and many times it reduces fading. Diversity involves downconverting and demodulating the received signal in parallel from the two or more antennas and then selecting the “best” signal by post detection decision criteria.
Referring to FIG. 1, a common technique used for antenna diversity in wireless systems is depicted. Each of the two antennas 5 and 10 is connected to a separate full channel receiver which includes a low noise amplifier (LNA) 15 and 20, a down converter 25 and 30, and a demodulator decoder 35 and 40. The demodulator decoders 30 and 40 each receive an analog signal either at an IF or a baseband frequency and then demodulate the analog signal to provide a digital signal which has also been error corrected. After decoding and error correcting the digital signal each demodulator decoder 30 and 40 delivers a decoded data stream, comprising data packets, along with error information related to each data packet of the digital signal. The digital signal from each of the demodulators 30 and 40 is delivered to the buffer 50, while the error information is delivered to diversity controller 45. The diversity controller 45 selects the best packet data stream from the one of the two demodulators 35 or 45 according to a predetermined minimum error criterion. The selected data packet is then provided at data output 55.
Referring to FIG. 2, a known technique used for beam forming in wireless receivers is depicted. The system of FIG. 2 utilizes vector combining, of amplitude and phase, of the signal received by each of two antennas 110 and 115. By amplitude balancing and adjusting the phase of the relative signals a shift in the direction of the combined antenna beam is achieved. The two received signals are typically combined in a beam forming RF module 80 that operates at the transmitted RF frequency. The signals from each of the two antennas 110 and 115 after been amplified by the attached low noise amplifier 70 and 75 is delivered to the beam forming module 80. The beam-forming module 80 includes a phase shift and amplitude control unit 85 and a RF combiner 90. The combined signal output by combiner 90 is a standard received signal at RF. The combined signal is then provided to a downconverter 95 and then to analog to digital converter (A/D) 100. The digital signal is provided to a demodulator 105 with a channel estimation capability. The demodulator 105 utilizes the estimation capability to control the beam forming RF module for optimization of the received signal and for error minimization. The two antenna elements as described in FIG. 2 are very common for low cost wireless applications, but the same concept can be used with more antennas. Therefore, the beam-forming scheme depicted in FIG. 2 can be implemented with two antennas or with a phased array.
The techniques described with respect to FIGS. 1 & 2 require a full receiving chain for each antenna. This is both expensive and increases the potential points of failure in a wireless receiver.